The treatment of prebaked carbon components of aluminium production cells, such as anode blocks, cathode blocks or cell sidewalls to improve their resistance to the conditions prevailing in the cell has already been proposed.
U.S. Pat. No. 5,486,278 (Manganiello et al) discloses treating a prebaked carbon-based anode of an electrolytic cell for the production of aluminium, in particular by the electrolysis of alumina in a molten fluoride electrolyte, over its sides and top to improve the resistance thereof to erosion and corrosion during operation of the cell by air and oxidising gases released at the anode, by immersing the anode in a boron-containing solution containing 5-60 weight % of H.sub.3 BO.sub.3 or B.sub.2 O.sub.3 in methanol, ethylene glycol, glycerin or water with a surface-active agent, e.g. at 80.degree. to 120.degree. C. After immersion, lasting up to an hour, the boron-containing solution is impregnated to a depth of usually about 2-5 cm over the top and side surfaces of the anode to be protected, producing a concentration of boron in the impregnated surface from 100 ppm to 0.35%. The same treatment can be applied to cell sidewalls.
It was found advantageous to carry out this treatment with a heated solution, but this involved heating of the anode, which consumed large quantities of energy. Attempts were therefore made to carry out the process at ambient temperature because no special heating equipment would be required. Low temperature application however required the careful choice of solvents and surfactant agents in order to reduce the treatment time as far as possible.
For prebaked anodes, only the top and top side surfaces need to be protected, so it was suggested to dip the anode upside down into the solution. But this is impractical when the anodes are fitted with rods for connection to a suspension device which also serves as a current lead-in. Furthermore, it is inconvenient to treat the anodes first and then fix the suspension rods.
To overcome this difficulty it would be possible to dip the anode in the treating solution with the rodded top side up, and protect the bottom part of the anode by blocking its pores with a fugitive agent that prevents impregnation with the boron-containing compound, and can be removed afterwards. This however entails additional operations and careful selection of the fugitive agent.
To speed up the process, it was suggested to assist the impregnation by the application of a pressure differential, by pressure or vacuum. However, no practical way of doing this was disclosed.
U.S. Pat. No. 5,534,130 (Sekhar) describes the protection of the cell sidewalls of aluminium production cells by impregnating them with agents based on aluminium phosphate. Again, it would be desirable to perfect ways of applying this method in an efficient manner.